1. Field of Invention
This invention relates to printed and integrated circuits and in particular to integrated CMOS tuned RF amplifiers.
2. Description of Related Art
Advances in CMOS technology has made it possible to integrate onto a single chip a communication system operating in a frequency range between 0.9 and 2.5 GHz which can be used in mobile phones and global positioning systems. An important aspect of the communication system is having a low noise amplifier (LNA). The prime objective of the LNA is to produce a signal at a given noise figure for a minimum expenditure of power. Since an LNA is a first stage of a receiver, the overall design will be dominated by the noise figure. When the gain of the LNA is large, noise contribution by subsequent stages becomes insignificant. Also a narrow bandwidth LNA provides increased immunity for low frequency base band signals.
In U.S. Pat. No. 5,861,778 (Louagie et al.) a low noise amplifier is based on a differential difference amplifier having a differential output with two pairs of differential inputs. This is done to minimize harmonic distortion. In U.S. Pat. No. 5,710,523 (Kobayashi) a low noise amplifier utilizing a high mobility transistor and a tunable heterojunction bipolar transistor is described. In U.S. Pat. No. 5,642,080 (Whang et al.) a low noise amplifier is described that includes an input impedance component consisting of an inductor and a capacitor. Two MESFETs are connected in cascade with a capacitor connected between them to enhance the stability of the amplifier. In U.S. Pat. No. 5,574,405 (Razavi) a low noise amplifier is combined with a mixer and synthesizer circuit to produce an RF system front end. The synthesizer shares power with the low noise amplifier to reduce power consumption. In U.S. Pat. No. 5,488,382 (Fenzi et al.) a low noise amplifier with high input impedance is described using a cascode pair of FET transistors. A feedback is provided from output to input to lower the Q of the antenna at the input of the amplifier, broadening the bandwidth of the detector and matching the antenna to the noise factor of the amplifier.
A common source tuned RF amplifier is frequently used as a low noise amplifier. Using a cascode amplifier as an input stage isolates an inter stage tank circuit from any input matching networks which can improve stability. An integrated inductor which forms a part of the tank circuit has a low Q (quality factor) which limits the Q of the LNA amplifier. The gain and Q of the LNA can be enhanced by adding a negative conductance at the output of the LNA to the tank circuit, but this usually means added active devices which results in more noise and more power dissipation.